1. Field of the Invention
The present invention relates to a flow measuring device that measures a flow of air, and, for example, relates to a flow measuring device that measures a flow of air suctioned into an internal combustion engine.
2. Description of Related Art
As shown in FIGS. 5A and 5B, a conventional flow measuring device 100 is disclosed to include an electrically insulating film 101, a heating element 103, and detection resistor assemblies 104, 105. The electrically insulating film 101 in provided in an air passage and has a planer shape that extends generally in parallel with the flow direction of air. Also, the heating element 103 is provided on a surface 102 of the electrically insulating film 101 for generating heat when energized. The detection resistor assemblies 104, 105 are provided downstream and upper stream of the heating element 103, and each of the assemblies 104, 105 includes multiple resistors. The flow measuring device 100 measures flow of air that flows through an air passage based on a difference or detection difference between detections of the upstream and downstream detection resistor assemblies 104, 105.
The conventional flow measuring device 100 includes detection resistor assemblies 104, 105, and each of the multiple resistors of the detection resistor assemblies 104, 105 is spaced away from one another in a flow direction (see FIG. 5A). The above arrangement is named as a series arrangement. In a case, where each resistor of each of the detection resistor assemblies 104, 105 is arranged as above, heat from the heating element 103 is less likely to be transmitted to the resistor positioned away from the heating element 103 compared with the resistor positioned closer to the heating element 103. As a result, the detectivity may degrade disadvantageously.
In contrast, for example, JP-A-2001-41790 (U.S. Pat. No. 6,557,411) and JP-A-2006-98057 (U.S. Pat. No. 7,270,000) describes that both of the resistors of each of the detection resistor assemblies 104, 105 are arranged symmetrically relative to a center line. In the above case, both resistors of each of the detection resistor assemblies 104, 105 are arranged side by side in a direction perpendicular to the flow direction such that the multiple resistors of each of the detection resistor assemblies 104, 105 are uniformly heated (see FIG. 5B). The above center line passes through a longitudinal direction center of the heating element 103 to extend in the flow direction of air. In other words, the center line passes in the flow direction through a center position of the heating element 103, which center position corresponds to a ½ length of the heating element 103 along the longitudinal axis of the heating element 103. However, when each resistor of the detection resistor assemblies 104, 105 is provided in a parallel arrangement as above, the detection difference may not be sufficiently obtained at the longitudinal direction center, at which the temperature distribution is most clarified. As a result, the detectivity may degrade disadvantageously.